Whereas in the past, material advances and improved professional manpower training largely determined improvements in the delivery of dental care, now the progressively sophisticated service demands of the public require the introduction of new technologies. To this end, an innovative technique has been developed to improve the quality and efficiency of dental diagnosis, treatment planning and evaluation, in addition to patient communication. In this technique, conventional dental impressions are digitized by a computer-controlled laser scanner. Subsequently these data are transformed by customized computer graphics software, so that the derived three-dimensional electronic models of the teeth and dental arches can be viewed on a computer terminal from any perspective or magnification. Additional software has been developed so that these models can be modified interactively to simulate the effects of treatment prior to actual commencement on a patient. In addition, these models can be readily transmitted to others for advice and/or treatment planning approval, stored on a computer disk for future reference, and integrated with other computer-derived diagnostic data (e.g. digital radiographic or periodontal assessments) thereby facilitating the development of `expert` systems.
Traditional hydrocolloid casts of the maxillary or mandibular dental arches are ubiquitous to many forms of dental service, due to difficulties in intraoral diagnosis, treatment planning and evaluation. Derived from alginate, silicone or rubber-base impressions, the main applications of study casts are summarized below.
(1). Orthodontics
(a) Diagnosis, PA1 (b) Treatment planning PA1 (c) Treatment progress evaluation PA1 (d) Treatment case records PA1 (a) Diagnosis, including the evaluation of wear and attrition facets PA1 (b) Treatment planning PA1 (c) Appliance design PA1 (d) Appliance evaluation PA1 (e) Treatment case records PA1 (a) Treatment planning PA1 (b) Treatment case records PA1 (a) Diagnosis PA1 (b) Treatment planning PA1 (c) Treatment case records PA1 (a) Diagnosis PA1 (b) Treatment planning PA1 (b) Treatment case records PA1 (a) Status of present dentition and treatment needs PA1 (b) Treatment options PA1 (c) Treatment progress PA1 (d) Treatment case records PA1 (a) Pre-authorization insurance company assessment PA1 (b) Medico-legal documentation. PA1 i. Laser probes using structured light principle, PA1 ii. Photogrammetric methods, PA1 iii. Laser range measuring probes with X-Y-Z tables, PA1 iv. Scanning laser range probes, PA1 v. Traditional mechanical coordinate measuring machines. PA1 (a). Simulation of major and minor orthodontic tooth movement facilitates objective appliance design and subsequent evaluation of treatment progress PA1 (b). Simulation of occlusal rehabilitation with/or without simultaneous orthodontic or prosthodontic treatment would facilitate discrimination between organic and functional occlusal disharmonies and enhance quality assurance in treatment planning PA1 (c). Simulation of cosmetic, restorative or prosthodontic treatment would enhance the potential for quality assurance of the adjacent hard and soft tissues PA1 (d). Simulation of potential orthodontic and/or periodontal relapse prior to treatment would provide quality checks in appliance design PA1 (a). Electronic storage of detailed dental arch measurements would facilitate instantaneous model referral for advice and consultation (Third Party, specialist etc.) PA1 (b). Dental arch three-dimensional simulations would provide excellent professional patient communication media to explain potential treatment options and their rationale for selection PA1 (a). As detailed dental arch dimensions can be stored on an office computer, the latter's increased utilization will facilitate service cost containment--The planned system for laser scanning and model simulations will be designed to utilize a standard dental office computer system PA1 (b). Enhanced quality assurance prior to treatment will reduce the potential for relapse and/or failure PA1 (c). By elimination the need for model storage, electronic dental arch data storage will facilitate record retrieval and archiving efficiency.
(i). dental arch evaluations, including relative tooth alignment and orientation PA2 (ii). functional occlusal analyses between maxillary and mandibular dentition's, including analysis of wear facets and attrition PA2 (iii). evaluations of maxillary and mandibular skeletal base relationships PA2 (i). timing of orthodontic treatment PA2 (ii). decision analysis between orthodontic and/or orthognathic surgical cases PA2 (iii). orthodontic appliance design PA2 (i). Fixed or removable appliance selection PA2 (ii). Pre-prosthetic treatment for remaining natural teeth PA2 (iii). Pretreatment orthodontic tooth realignment PA2 (i). Abutment tooth selection PA2 (ii). Identification of potential rest seat and clasp locations PA2 (iii). Clasp design and abutment tooth location PA2 (iv). Pontic design PA2 (i). Complex cavity design PA2 (ii). Restorative material selection
(2). Prosthetic dentistry
(3). Restorative dentistry
(4). Pedodontic dentistry
(5). Periodontics
(6). Patient communication
(7). Third Party communication
Yet reliance on study casts has hampered significant improvements to dental service quality and cost efficiency. For instance, visual appraisals of their morphologic form primarily hinge on the biased experience of the observer, whereas the alternatives of ruler, protractor or grid measurements are too restrictive to offer significant improvements to their evaluation. Whereas study cast evaluations are necessary to compensate for difficulties with in situ appraisals of the teeth and dental arches, only a fraction of their component data can be delineated by existing evaluative techniques. Dental diagnosis, treatment planning and evaluation therefore remains largely subjective, and this restricts their objective appraisals required for significant improvements in service quality assurance and cost containment. The primary deficiency of study cast evaluations stems from difficulties in their measurement.
Other deficiencies arise from difficulties in their storage and retrieval due to their physical bulk. Traditional study casts are also static and cannot be readily manipulated, which restricts their applications when evaluating potential treatment options and their presentation to patients. For example, cutting and repositioning teeth on a cast is conventionally used to simulate potential orthodontic realignment options, whereas trial wax-ups on a study cast are often components of complex restorative treatment planning, including abutment tooth selection and pontic design. In cases requiring complex occlusal rehabilitation, spot grinding or other forms of adjustment are often simulated first on study casts prior to commencing treatment on an actual patient. But all techniques involving traditional study casts are relatively crude, subjective and time-consuming, primarily due to difficulties in their precise measurement.
The complex morphologic forms of teeth and dental arches are difficult to measure with any degree of precision. Nevertheless, many techniques have been developed to measure individual or groups of teeth very accurately as a component of CAD/CAM technology applied to dentistry.
Well established in the aerospace, automotive and large manufacturing industries, computer aided manufacturing and computer aided design (CAD/CAM) have significant potential for improved quality and cost efficiency when applied to dentistry. Unfortunately the lack of accurate measurement techniques restricts their application to small complicated biological bodies such as a tooth. Since the accuracy requirements for dental diagnosis, treatment planning and evaluation are similar to precision manufacturing standards, data acquisition is the principal deficiency of current CAD/CAM dental applications. The five measurement techniques reported for CAD/CAM dental applications thus far include the following:
The CEREC System which has been developed by Brains-Brandestini Instruments of Zollikan, Switzerland (Moermann and Brandestini 1986) and is currently marketed by Siemens Dental Division, FRG (Siemens 1989) and Dr. F. Duret (1988) are both employing a specially designed hand-held probe to measure the three dimensional coordinates of a prepared tooth. The measurement probe design embraces the structured light principle. But in order to eliminate possible image artifacts from dark garnishes on the tooth's surface, saliva, debris etc., a talc and titanium oxide powder mixture combined with a wetting agent must be applied to the area to be measured. Methods to control powder thickness and the resultant masking effect on the fine cavity preparation details have yet to be reported. Due to difficulties in data acquisition and processing from the in situ use of a hand-held optical probe, a modification is using a mechanical arm to hold the probe and a partial study cast of the prepared tooth is actually measured.
The Photogrammetric principle to measure the profile of a prepared tooth cavity is a component of the proposed system developed at the University of Minnesota (Rekow 1987). A pair of stereo images are recorded on the standard film using a modified 35 mm camera with a single-rod lens attached to a laryngopharyngoscope. Major difficulties of this system include saliva and other image contaminants and the automation of tooth profile measurements from stereo images.
The commercial coordinate measuring machine (CMM) has been proposed and a very few examples have been manufactured and used in research establishments. This uses a laser range probe for non contact measurement of a cast model of the teeth of the patient. It has data acquisition rate of only a few points per minutes and more than 12 hours is required to measure a complete cast. An optical CMM (Yamamoto, 1988) with data sampling speed of 25 ms (i.e. 40 data points per sec.) has been reported with measurement accuracy in the range of 100 mm. Approximately 1 hour is required to measure an impression. These devices are therefore of little practical value.
The scanning laser probe described by Rioux (1984) has very high data acquisition rate but is unfortunately very expensive. This has not been proposed for dental modeling but only for industrial operations. This device uses a highly complex moving mirror arrangement to effect the scanning and this leads to the very high cost which makes it completely impractical for the present requirements.
Using traditional coordinate measuring machine or a miniature mechanical arm to capture data from stone dies has been proposed by many researchers (Rekow, 1992). Major disadvantages of a mechanical probe include slow data acquisition speed and limited measurement resolution. Surfaces which have radii of curvature or depression less than the mechanical probe tip radius cannot be detected. With probe tip diameters less than 0.5 mm, their mechanical integrity difficult to maintain, in addition to their potential for surface damage.
As all reported measurement systems suffer from serious deficiencies, none can be considered a viable clinical instrument. Capital costs (laser scanning probe), difficulty in usage (mechanical probe), inaccuracy (optical probe and mechanical probe) or speed (mechanical CMM) limit their routine application for diagnosis, treatment planning or evaluation.
There remains therefore a high requirement for a dental modeling system in view of the following major advantages:
(1). Prior treatment planning simulation
(2). Communication
(3). Overhead cost reduction